The Heat is On

I’ve been chopping down some old man pine for firewood. The trees are getting on for a hundred years old, probably planted around the same time as our farmhouse was built. They’ve done a great job provid­ing shelter, but their shade is not welcome on a corner of our young vineyard and their roots are out-competing the vines. As the gnarly old trunks are chainsawed into rounds for splitting, the slabs of wood reveal the progres­sion of the trees’ lives. Wide rings mark years of rapid growth, narrow ones mark years when the tree struggled. They show warm years, cold years, wet years and dry years. They sum up the weather, integrate it, and present the informa­tion to be read by those with the key.

Human memory is much more fallible. If I asked the families who’ve lived here about the weather they experienced over the last century, they’d remember nor’west gales that uprooted trees, the great snowfall of 1945, big floods and testing droughts, heavy rain that brought landslips and gullying that ate into paddocks. They’d remember warm summers and cold winters, and perhaps be able to show me rainfall records. But if I were to ask them how the climate had changed, they would be unreliable witnesses. We remember the big things, but we’re not designed to record subtle changes in long-term averages.

Climate change is about variations in small numbers, and the power of small numbers is something we underestimate at our peril.

Car­bon dioxide, the greenhouse gas that’s getting us into trouble, only amounts to about 385 parts per million of the gases in the atmosphere—not a large amount. But that’s a third more CO2 than 150 years ago, enough to have caused, and continue to cause, significant changes in local and global climate. Twelve of the 13 years from 1995 to 2007 have been the warmest since 1850, while 1998 and 2005 are the warmest years since records began. Over the last 100 years, New Zealand’s temperature has increased by 0.9ºC.

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Earlier this year, NIWA released the latest projections for New Zealand’s climate over the next century. To produce these, NIWA’s research­ers used the results of the Intergovernmental Panel on Climate Change’s (IPCC) Fourth Report, published last year, which provides a broad-brush picture of the changes likely to take place around the globe. To get down to the level of Paraparaumu or Kai­teriteri, the projections have to be “downscaled”. NIWA used both statistical and dynamic downscaling; the former uses local climate data and develops statistical relationships between, say, rainfall in Hokitika and what’s happening at the larger, global-model scale. Dynamic downscaling uses NIWA’s new regional climate model, taking the global model projections for the New Zealand region, and using those to calculate what hap­pens here on a much finer scale.

NIWA used a middle-of-the-road emissions scenario, the IPCC’s A1B projection, a projec­tion that assumes strong economic growth, a global population of 9 billion by 2050 (falling thereafter) and the rapid introduction of clean energy technology.

According to this regional climate model, for at least the first half of the century we will experience invigorated westerlies, increased rainfall in the west, and increased dryness in the east. In short, it’s today’s climate on ster­oids, and warming all the time.

Our annual average temperature is likely to increase by about 1°C by 2040, and 2°C by 2090. It doesn’t sound like a lot-temperatures can change by tens of degrees in a single day. But that 2°C change is a bit like moving Wel­lington to Auckland. Dunedin will be more like Marlborough, Auckland a bit more like Sydney. It’s as if Maui had upped anchor and sailed his waka a few hundred kilometres closer to the equator.

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The results suggest that New Zealand, relatively speaking, won’t be too badly off. We’re surrounded by big cool oceans, which take a long time to warm up, and our 2ºC warming is well below the global average increase—expected to be about 3ºC. Other places, like the Arctic and the centres of the large Northern hemisphere continents, will warm up by much more. But small numbers can have a big impact: severe droughts could become twice as frequent, rainfall might be heavier, in turn causing more flooding and erosion. There will be less frost and more days that are unusually hot.

For instance, Auckland has about 21 days a year over 25ºC, but by 2090, its residents will experience 60 days of temperatures over 25ºC. If we fail to curb emissions, there could be as many as 80 days a year that exceed 25ºC. And, as mentioned and as you might expect, it won’t get as cold. The North Island (excluding the mountains) currently has 30 to 40 frost days a year. That figure could drop to somewhere between five and 15 days a year.

There will be other effects. Warmer air can hold more water vapour, and water vapour (apart from being an important greenhouse gas) is also often described as the “fuel” for weather systems. As the century passes, the warming atmosphere will hold more water and the flow of water around the atmosphere, land and ocean will speed up. Rainfall in New Zealand will be heavier, shorter and sharper.

The usually sheltered Waitemata Harbour became a frothing cauldron as one intense storm after another (labelled by Metservice a one-in-10-year event) wreaked havoc in July. A notable increase in extreme weather events over recent years may be a foretaste of things to come as regional climates are predicted to manifest more intense versions of present patterns.

Warming also brings shorter snow seasons in the mountains, but could bring more snow­ fall, at least as long as the snowline remains below the mountain-tops—NIWA is planning work with a specialised snow model to tease out the balance between rising snowlines and increasing snowfalls. Any changes will impact on our rivers and hydro storage, as most of the winter precipitation in the mountains around the hydro schemes in the South Island falls as snow. This means that lakes that have low levels at the beginning of winter (as they did this year) will only start to fill again when spring melts the snow pack. Warmer winters will bring more rain, and help to alleviate the dry-winter problem—and more rain in the west will also probably provide more spillover rain events to top up the lakes at all times of year.

A warming climate will also have a consider­able impact on ice levels in the Southern Alps. Over the last 30 years, the volume of ice in the Alps has reduced by 11 per cent. While the fast-moving Franz Josef and Fox glaciers on the West Coast should do well in the early part of the century, as strengthening westerlies bring more snow to their névés on the Main Divide, in the long run there will be more rain rather than snow higher up, causing the glaciers to retreat.

The east coast of both islands will probably suffer more droughts, with severe droughts­ which occur once every 20 years in the cur­rent climate—occurring twice as often by the century’s end. A one-in-20-year drought is a serious event for farmers, and for the national economy.

A drought every 10 years will barely give farmers a chance to recover before the next strikes, especially if the periods in between are characterised by low rainfall.

More rain up on the Main Divide will benefit river flows in Canterbury, but it will be drier on the plains. Longer irrigation seasons will be needed to compensate and the tempta­tion to turn to the rivers for more water will be hard to resist.

Rainfall is expected to decline in the north and east of the North Island and down the east coast of the South Island over the first half of the century, but strengthening westerlies will bring more rain to the south and west of the South Island. All of the model runs used by NIWA suggest that the westerlies will be more vigorous in winter and spring, but later in the century there are signs that the subtropical high-pressure zones will move far enough south to bring summer rain in the east. Both the north and east of the North Island, and perhaps the top of the South, will experience more northeasterlies. Gisborne and Hawkes Bay will have more rain, while Taranaki and Buller district might have less—a pattern very similar to that seen over last summer.

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And when it rains, it will probably pour. Every 1°C of warming will mean that heavy rain events drop at least eight per cent more water; one in 100-year rain events could become twice as common.

Beyond the obvious weather-related changes, there is another aspect of global climate change that could have a dramatic impact on New Zealand. Sea level rise over the last 100 years has been relatively modest—about 16 cm in this part of the Pacific—but as the planet warms it will accelerate, presenting significant chal­lenges to low-lying coastal properties as well as expensive city infrastructure in Auckland, Wellington and Dunedin.

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At face value it seems as though we should be able to adapt as gradual warming takes place over several generations. But the rate of temperature increase has to be considered in its broader context. In the depths of an ice age, the global average temperature is about 5°C lower than it is at present. When an ice age ends, it takes the planet about 5000 years to warm up, usually at 1°C every 1000 years. Now we’re projecting 2°C increase in just 100 years, a rate of warming 20 times faster than any recent global climate change.

The speed of the change will have significant impacts on natural ecosystems. Communities of plants and animals that are adapted to the present climate will respond to warming at dif­ferent speeds.

Our agricultural industry has the capacity, and technologies, to adapt: if drought becomes a problem, then irrigation (if water is available) will get us through; if reduced winter chilling reduces crop yields, as will happen to kiwifruit in the Bay of Plenty by the middle of the century, a grower can change his crop to something that prefers more warmth, such as citrus, or move to follow the ideal kiwifruit climate. (It is more likely that, in the immediate future, farmers will be more preoccupied with year-to-year climate variability than the underlying slow warming.)

There will be winners and losers. Southland and South Westland will benefit from extra warmth and reliable rain, and in the long run will warm least, putting less stress on agri­culture and ecosystems.

While drought and competition for water will increase costs for east coast dairy farmers, their colleagues down south will be sitting pretty.

New Zealand’s wine industry should also do well. Most grapegrowers would welcome a little more heat to help ripening and boost yields, and a reduced risk of frost at key periods, such as flow­ering, will cut crop losses. If a vineyard warms to the point that grape quality falls, existing vines can be grafted over to new, warmer climate varieties relatively quickly. Growers in the North Island could switch to red grapes such as shiraz, while cool-climate varieties like sauvignon blanc and pinot noir could move south, perhaps into Southland.

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The picture we have of the future of New Zealand’s climate offers some clues on what can be done to prepare and adapt. But these projections are only of one, very local picture, and close-ups never tell the entire story.

There are political uncer­tainties, uncertainties about whether emissions reductions will be achieved quickly enough, uncertainty about how the cli­mate system will respond as it ac­cumulates energy. Evidence from ice cores in Greenland suggests that dramatic swings in climate are possible in as little as a year or two.

There are risks we can’t yet quantify, but they are real nonetheless and could make any comfortable assumptions about gradual warm­ing in New Zealand completely irrelevant. Arc­tic warming, for instance, could cause climate and economic dislocation on a global scale. It is not known what effect the melting of the West Antarctic ice shelves will have on inland gla­ciers. NIWA’s guidance to local authorities on sea level rise is that they should allow for up to 0.8 m over the next 100 years, reflecting recent research which suggests that the great Antarc­tic ice sheets could melt faster than anticipated only a few years ago.

But if climate change does continue to be gradual, then New Zealand will be reasonably well-placed to adjust. Auckland might be as warm as Sydney, but it won’t be as hot as Brisbane. The key to adapting to a warming world is to be smart enough to look ahead, and resilient enough to be able cope with change as it happens.

The Air up There

New Zealand’s climate is just part of a much larger system. The top of the North Island pokes up towards the subtropics, and the South Island digs down into the bands of strong westerly winds that circle around Antarctica; what happens in those two regions of the atmosphere de­termines what happens over us.

On an even larger scale, the earth’s weather is driven by solar en­ergy, most of which hits the planet in the tropics. At the equator, warm air rises up to the top of the troposphere (the bottom layer of the at­mosphere), and then spreads out towards the poles, cooling and finally sinking towards the surface, creating a band of high pressure in the subtropics called a Hadley cell.

Over both poles cooling air sinks towards the surface and then spreads out to the north forming a Polar cell. In between the Polar and Hadley cells, a band of strong westerly wind circles the planet. As the Hadley cell becomes larger with a warming climate, subtropical high-pressure bands will be pushed farther south. In fact this is already happening; the subtropics have expanded by some 250 km either side of the equator over the last 30 years. Fortunately the Southern Ocean will absorb much of the heat and warm more slowly than anywhere else on the planet, which will help to keep us cool.

Big cycles in the atmosphere and ocean over the whole Pacific can have a dramatic impact on our weather and climate. The best known of these, the El Niño/Southern Oscillation, ENSO for short, involves changes in the position of warm water in the tropical Pacific. In an El Niño, the easterly trade winds in the tropics are weaker than normal, and warm water stays close to the American coast. This causes changes in tropical pressure patterns and therefore also the flow of weather systems. In New Zealand, El Niño usually delivers more westerly winds, cooler-than- average weather and drought on the east coast of both islands.

La Niña is the opposite phase of the ENSO cycle. This occurs when the tropical trade winds are stronger, so the warm ocean water piles up in the western Pacific, and colder water drawn from depths spreads over the eastern tropics. La Niña is often associated with warmer than aver¬age weather and more northeasterly winds, as happened last summer.

The ENSO cycle is also affected by longer-term ocean/atmosphere cycles in the Pacific—in particular, the Interdecadal Pacific Oscillation (IPO) and its close relative the Pacific Decadal Oscillation (PDO). These cycles have “warm” and “cold” phases that can last between 15 and 30 years. During the cold phase of the IPO, La Niñas are more frequent and stronger while El Niños become more frequent during the warm phase. There are signs that the IPO has recently flipped into its cold phase, which might slow down the rate of warming on a global level, but still result in a run of warm summers.

New Zealand will also be affected by what happens in Antarctica, which is more complex. The ozone hole in the upper atmosphere is currently counteracting warming, but as it recovers over the next 50 years, the continent will begin to warm more rapidly. As the pole re­mains cold while the rest of the hemisphere warms, the temperature gradient across the Southern Ocean will increase, resulting in more intense westerlies, which have already been observed.

The top graph shows annual temperatures since the beginning of the last century extrapolated into the future. This isn’t a forecast that makes predictions for individual years but it does illustrate the effect of natural climate variability. By 2040, an average year will be as warm as one we would currently consider hot while a hot year will be hotter than anything we’ve experienced in the last few hundred years. By 2090, a cold year will be the same as an average year in the 2040s, and a warm year will be scorching.

The lower graph shows how temperature increases over time. Like the historical data up to 2000, the blue line shows wiggles up and down, reflecting natural variability. By 2040 the annual average temperature will have increased by 1°C, and by 2090 by 2°C. The rate of increase is faster than we’ve seen over the last 100 years (the black dotted line shows what that would look like projected forward), but is less than the global average of 3°C by the end of the century. Yellow bars give the range of annual averages for the full range of IPCC scenarios, showing that under the highest scenarios, the increases could be as high as 2°C by 2040 and 5°C by 2090. Al­ternatively, if the world achieves aggressive emissions reductions, the rises we experience will be relatively modest.